1. Field of the Invention
The invention relates to a laser apparatus with a gas circulation and excitation system, which is designed according to the gas transport or convection principle.
2. Prior Art
Power, amplification and efficiency of molecular lasers, in particular CO.sub.2 lasers, decrease with increasing temperature of the laser gas. The decrease in efficiency is due to the fact that with rising temperature the line width becomes larger, the excitation energy distributes among an increasing number of rotational lines, the number of deactiviating collisions increases and the population of the laser end level increases by thermal excitation, which results in a decrease in inversion of the individual transitions (K. Gurs, Laser 75, Optoelectronics, Conference Proceedings, pp. 30-37).
For this reason methods have been developed which carry off the heat together with the laser gas by circulating and cooling the gas. Lasers appropriate for this method consist of an active region in which the gas is excited, with a separate (downstream) or integrated optical resonator, of the gas transport system with a built-in cooler and a pump. As large amounts of heat have to be carried off, large gas volumes have to be transferred by pumping. The respective lasers are large and expensive, and their applications are limited.
All of the CO.sub.2 lasers with output powers of more than 1 kW which are on the market operate according to this principle of gas transport or convection laser. It was first described in 1969 by Tiffany, Targ and Foster.
A conventional gas transport laser uses a high-power blower, e.g., a fan or a Roots pump, for rapid gas circulation. Depending on the arrangement of the laser resonator and the direction of gas flow, a distinction is made between tangential-flow lasers and axial-flow lasers. Both of these lasers have specific advantages and drawbacks. In the case of the tangential-flow laser, the pressure loss is relatively small because of the large flow cross section. Therefore, it is possible to maintain the necessary flow rate by means of a smaller blower than in the case of the axial-flow laser. In the case of this latter laser type it is more difficult, however, to achieve homogeneous discharge and emission in the fundamental mode. For this reason the emission of the axial-flow laser can in general be better focused.
Convection lasers, in which the gas flow is passed in axial direction through glass tubes serving as discharge vessels, have a better radiation quality which is partly due to the radially symmetric discharge conditions; their radiation can be better focused. A sufficiently high flow rate can, however, only be maintained by means of a relatively high pressure gradient within the laser tube. As a result, the discharge conditions are not uniform along the tube. A strong and heavy circulation blower (Roots pump) is necessary.
An improvement of the axial convection laser is described in German OS No. 29 16 408. This design provides for helical circulation of the gas mixture within the discharge tube by suitably designed, water-cooled baffle plates. Its passages through the active zone of the laser are only of short duration; between the passages, there is sufficient time for the heat absorbed to be dissipated. The helical baffle plates are provided with bores lying on a line parallel to the axis. Through these bores the gas discharge burns to excite the laser mixture. This arrangement has the advantages that only a relatively low circulation rate is necessary and that the longitudinal arrangement of the laser resonator ensures a high mode quality. It is difficult, however, to keep the gas discharge in place, as it is easily blown out of the resonator by the gas flow. Favorable operating conditions can be maintained only with specifically defined discharge parameters, so that the intensity of this laser type cannot be satisfactority controlled.